A number of processes for concentration of sulphuric acid up to concentrations of 96%-98% H.sub.2 SO.sub.4 are known and summarized in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A25, pages 687-690, Springer Verlag 1995. Such process include:
Boiling the sulphuric acid under vacuum, typically in a circulating evaporator at 15-50 mbar absolute pressure in the flash evaporator vessel. The vapors are fractionated in a column, whereby most of the H.sub.2 SO.sub.4 is separated from the H.sub.2 O vapor and returned to the flash vessel. Vacuum concentration of sulphuric acid is based on well known distillation principles and 98% acid strength can be achieved, but investment and operating costs are high.
Concentration of sulphuric acid at atmospheric pressure is known in various variants, for example:
The Chemico direct-fired drum concentrator in which hot flue gas from combustion of oil or gas is passed through a number of vessels in which the flue gas is passed through an immersion tube into the acid to be concentrated. The acid flows countercurrently with the gases through the vessels arranged in cascade fashion, as shown in FIG. 45 page 688 in Ullmann. The most serious draw-back of this process is severe corrosion problems and the formation of a large amount of offgas with high content of sulphuric acid fumes that are difficult to remove before the gas can be emitted to the atmosphere. These problems increase with the attempted strength of the product acid and 98% H.sub.2 SO.sub.4 can hardly be achieved.
The Pauling-Plinke process in which the acid is boiled in an indirectly heated pan, and the vapors are fractionated in a distillation column. Corrosion problems are severe and 98% H.sub.2 SO.sub.4 are difficult to achieve.
The Bayer-Bertrams process, described in detail by Kung and Reimann in reference (143) in Ullmann, in which the acid is concentrated in a number of indirectly heated, circulating evaporators connected in series. 98% H.sub.2 SO.sub.4 can only be achieved by the use of vacuum in the last one of the circulating evaporators.